I suspect Bulgaria is not quite as aggressive as, say, UK or USA, on discovering an overstayer... but it's definitely not recommended!
The allowance is "up to 90 days in any 180-day period". Which, as you already realized, you have now exceeded.
As you haven't yet applied for your D visa, it's better to stay out of trouble until then. As this application must be done in London (or, at least, outside Bulgaria) you have to leave anyway. So maybe sooner rather than later is better!
So the official position is easy to figure out. The question is what to do? I have some thoughts:
- Probably the safest is just to hotfoot it out of Bulgaria ASAP. Either by flying back to UK and making a new plan for 3 months time. Or drive across the border to Serbia/Turkey (cheap, close, not EU) and hide out there for a few days/weeks while you devise a cunning plan. This option will show you whether Bulgaria has a sophisticated IT system that automatically counts your days in the country. If they just look at your passport and don't complain (probably more likely when driving out than flying out), that's a nice bonus. If so, you can try to come back in a few days later, and see if they are any more careful when you return. If they are, they will just say you can't enter because you already used your 90 days allowance (no big deal, you stay in Serbia/Turkey, and fly home from Belgrade/Istanbul).
- If they stamped your passport when you came back from Egypt, then I suppose you have a plausible excuse with the "Oh, I thought it was 3 months from entry date" line. Ignorance is no defence if they get all legalistic with you. But maybe if delivered with charm, to a friendly immigration officer (if there are any), maybe he only gives you a slap on the wrist, and tells you not to do it again. And maybe it's no worse to overstay by 2 months (to 20th Feb) than to overstay by 5 days. If you felt that way, maybe you could quickly get married so when you leave you also have a nice Bulgarian marriage story, and you're on your honeymoon, so they have to be even more heartless to give you a hard time. :-)
- You get on the phone to a Bulgarian immigration attorney immediately. And you ask exactly how bad overstaying is. And if there's an easy application for extension that he can file. And whether he recommends you get out immediately, or file for an extension, or get married first and overstay longer. There are a couple of lawyers who post here occasionally. And I can give you contact info for mine if you private message me.
Financially, a Wizzair/Ryanair flight back to the UK (assuming you have your home there still) is probably the cheapest option, and the least amount of overstay. And hopefully no hassle when you come back in April, and you have plenty of time to get married on your next trip. An attorney is the most expensive option, but if he can get an extension, or he thinks overstaying by a couple of months is no big deal, then you could be applying for your D visa a few months sooner.